List of selling prices in Paper Mario
From the Super Mario Wiki
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In Paper Mario, Mario can sell items from his inventory to shopkeepers in the nine item shops around the Mushroom Kingdom. Most items can be sold to the shopkeepers for a set price at every shop, however, some items will sell at a higher price depending on the location of the shop and the item itself. Mario can make a profit if he purchases an item at a low price from one store and sells it to another that will buy it from him at a higher price. This page lists every item and how much it can be sold for at each shop.